The October 2006 “PQ Corner” article, written by Thomas S. Key of EPRI, on the topic of “Avoiding the Pitfalls of Powering CNC Equipment” was very good; however, he made a couple of incorrect statements about NEC rules.
For example, the middle paragraph (on page 24) states the purpose of the grounding electrode conductor and grounding electrode. Statement 1 about path to earth for lightning is correct, Statement 2 about path to the source is false [since the GEC and GE is a connection to the earth, and the earth is not a low-impedance path to the source, see 250.4(A)(5)]. Statement 3 is okay, but it's really not clear what he is trying to say.
Per 250.4(A)(5) Effective Ground-Fault Current Path, “Electrical equipment and wiring and other electrically conductive material likely to become energized shall be installed in a manner that creates a permanent, low-impedance circuit facilitating the operation of the overcurrent device or ground detector for high-impedance grounded systems. It shall be capable of safely carrying the maximum ground-fault current likely to be imposed on it from any point on the wiring system where a ground fault may occur to the electrical supply source. The earth shall not be considered as an effective ground-fault current path.”
Per 250.2 Effective Ground-Fault Current Path, “An intentionally constructed, permanent, low-impedance electrically conductive path designed and intended to carry current under ground-fault conditions from the point of a ground fault on a wiring system to the electrical supply source and that facilitates the operation of the overcurrent protective device or ground fault detectors on high-impedance grounded systems.”
In the top left paragraph (on page 26), the statement that the supplementary electrode is required to be bonded to the building grounding electrode system is false — I used to think this was true, too. The 2005 NEC made changes to 250.54 to make this clear that when a supplementary electrode (soon to be called auxiliary electrode in the 2008 NEC) is installed it is NOT required to be bonded to the building GES. Per 250.54 Supplementary Grounding Electrodes, “Supplementary grounding electrodes shall be permitted to be connected to the equipment grounding conductors specified in 250.118 and shall not be required to comply with the electrode bonding requirements of 250.50 or 250.53(C) or the resistance requirements of 250.56, but the earth shall not be used as an effective ground-fault current path as specified in 250.4(A)(5) and 250.4(B)(4).”
At the bottom of page 26, the text about the isolated ground rod Code violation is wrong. On page 28, all of the text about bonding supplementary electrodes to the building GES is not true.
— Mike Holt, Mike Holt
Enterprises, Leesburg, Fla.
Author's Response: “Thanks to Mike Holt for updating us on the recent Code change. His comments helped us to clarify some key points in the article.
On page 24, bottom middle paragraph, regarding our discussion on the intended benefits of the grounding electrode system, I agree with Mike's point that the electrode system's connection to earth should not be considered as an effective ground-fault current path.
A key point in the article is to maintain the NEC defined (250.2) effective ground-fault current path, and not to rely on a supplemental ground rod for this path. Even so, normal bonding of the effective ground-fault path to the grounding electrode system does enable that system to play a positive role in clearing ground faults, without relying on earth return. For example, the grounding electrode connection can end up in the ground-fault path when there is a fault external to the grounded equipment frames and raceways. This occurs when a portable appliance or extension cord becomes faulted to the floor or building steel. While not the ideal ground-fault path, the grounding electrode system, nonetheless, is part of, and assists, the ground fault path.
In the top left paragraph on page 26, Mike makes a good point — since the 2005 Code change, a supplemental ground rod is not required to be bonded to the building grounding electrode system. I believe the rationale in this change is, since the rod is supplemental it is not required for safety. However, it is required that electrically energized equipment with this supplemental ground rod shall also be effectively connected to the grounding conductor in its branch circuit.
The text on the bottom of page 26 needs to be clarified. Per the 2005 Code change, the supplemental ground rod is not required to be bonded directly to the rest of the building grounding electrode system. However, as stated above, the equipment must be grounded via its branch circuit and not rely on the supplemental ground rod as an effective ground fault current path. Presence of a PVC bushing, as shown in Fig. 3, is good reason to suspect a problem and to check if the equipment does in fact have a grounding conductor that returns to the source via the branch circuit.
— Thomas Key, EPRI,
Upon Further Review
The January 2007 “Product of the Month” story, “Wireless Switch Sheds Reliance on Battery Power,” featured a “unique” battery-free, wireless technology product offered by Ad Hoc Electronics. Upon further review; however, we discovered the item isn't as unique as we first thought. There's another battery-free, wireless switch manufacturer operating in the states. PulseSwitch Systems of Norfolk, Va., has been selling its patented Lightning Switch wireless and battery-less switch controls in North America since 2004.
For more information, visit www.lightningswitch.com.
— EC&M editorial team